A Kort nozzle is a conically tapered tube or duct in which the propeller of a ship is placed. The tube is a so-called nozzle ring and forms the wall of the Kort nozzle. Due to the taper of the nozzle ring towards the stern of the ship, the Kort nozzles can transmit an additional thrust to the ship without the output having to be increased. Besides the propulsion improving properties of the Kort nozzle, pitching by rough sea is thus reduced so that by sea disturbance the lost of velocity can be reduced and the directional stability can be increased. Since the inherent resistance of the Kort nozzle increases approximately quadratically as the speed of the ship increases, its advantages are effective in particular for slow ships which have a big propeller thrust (for example tugboats, fishing vehicles, etc.).
Besides fixed Kort nozzles behind which normally a rudder is placed in flow direction for the control of the ship, there are so-called “Kort rudder nozzles” for which the Kort nozzle is rotatable about the rudder axis of the ship which is in vertical direction. For this purpose, bearings are normally provided on the upper and lower side of the Kort nozzle on the outside of its wall (nozzle ring) for the rotatable positioning. In contrast, the propeller is still fixed so that the Kort nozzle also rotates around the propeller. Frequently, the Kort nozzle is connected with the rudder post and positioned in the rudder heel. It is normally swivelable about a vertical axis of rotation or about the rudder axis by approximately 30° to 35° to both sides, the starboard side and the port side. Thus, the Kort nozzle is a combination of propulsion improving means and rudder since a rudder effect is achieved by the excursion of the propeller jet at an angle to the ship longitudinal axis. For excursed rudder nozzles, the stern of the ship is pushed by the jet reaction propulsion.
FIG. 5 illustrates an embodiment of a Kort nozzle 200 positioned rotatable about the rudder axis of a ship with a fixed propeller placed therein as it is known from the prior art. The Kort nozzle 200 is placed around the fixed ship propeller 210 of a ship (not represented here). Here the Kort nozzle is pivoted under an angle α of approximately 30° about the ship longitudinal axis 220. The arrow 221 represents the flow direction of the sea water or salt water. A fixed flap 230 is provided in flow direction behind the propeller on the Kort nozzle 200, through which the flow properties of the Kort rudder nozzle are positively influenced. Due to a reduced wall thickness, the inlet area 201 (with respect to the direction of the flow passing through the Kort nozzle 200) is configured widened with respect to the remaining area of the Kort nozzle 200. This means that the inner diameter of the inlet area is bigger than the inner diameter in the remaining area of the Kort nozzle 200. The water flow through the Kort nozzle 200 is increased which in turn increases the propulsion efficiency of the Kort nozzle.
Comprehensive calculations, tests and simulations of the applicant resulted in that, for certain twisting angles of a conventional Kort nozzle, swirls or recirculations of the flow form in the area directly aft of the propeller. These recirculations or swirls have a disadvantageous effect on the power of the Kort nozzle. They develop in particular directly behind the propeller in the side area of the propeller to which the Kort nozzle is turned. Due to the recirculations the flow rate of the water flowing through is considerably reduced in this area so that the driving power of the Kort nozzle is reduced. Since the recirculations occur only in a locally limited side area and the flow runs substantially laminarly in the other areas as usual, considerable vibrations which can be transmitted to the hull of the ship and which have also a disadvantageous effect occur. Reference is made in this regard to the FIGS. 6a and 6b for illustrating this problem.
FIG. 6a shows schematically the top view of a cut Kort nozzle 200 as it is known from the state of the art. The arrows in FIGS. 6a and 6b constitute the course of the flow. The ship propeller 210 is drawn only schematically for reasons of clarity. For this Kort nozzle 200, contrary to the Kort nozzle of FIG. 5, a movable or swivelable flap 231 is placed in flow direction behind the propeller 210. The Kort nozzle 200 is swiveled with an angle of approximately 15° with respect to the ship longitudinal axis. The rear part of the wall 202a of the Kort nozzle 200 has been rotated against the flow direction, i.e. to the propeller 210, while the opposed part of the wall 202b has been rotated with the flow direction accordingly.
The lower part area of the Kort nozzle 200 which is marked in FIG. 6a is depicted enlarged in FIG. 6b. It can be recognized therein that, due to the angular position of the Kort nozzle 200 with respect to the propeller 210 or to the ship longitudinal axis 220, a swirl or recirculation of the flow forms in the outer edge area in flow direction directly behind the propeller 210. Due to this recirculation, the mean flow rate in the main flow direction 221 is reduced to a minimum in this local area. Measurements and simulations in this area showed that there is a mean flow rate of 0.2 to 2 m/s in the main flow direction 221. Compared to this, the mean flow rate is situated within a range of 12 to 16 m/s in the area between the flap 231 and the wall area 202b. 
The water which flows laminarly outside along the wall 202a flows around the rounded-off edge of the wall of the Kort nozzle end area 203 to the inside and hits there the flow produced by the propeller 210 which is directed in the main flow direction 221. A part of the outer flow is directed to the inside against the main flow direction 221 and flows on the inner side of the wall 202 against the main flow direction 221 to the area behind the propeller 210 and from there back again through the propeller 210. Thus, a local circulation or recirculation of the flow is formed and the mean flow rate in the main flow direction 221 in this area is around zero. Therefore, the disadvantages described above occur.